This invention relates to a Light Distancing and Ranging scanner (LIDAR) and particularly to a LIDAR scanner employing a rotating polygon mirror for directing the scanned and received light.
Three-dimensional imaging systems have improved the performance of a wide variety of automation systems. While three-dimensional images can be computed from multiple sets of two-dimensional images, this approach is more complex and less accurate than systems which collect images that are fundamentally three-dimensional. In general, these systems collect large amounts of three-dimensional coordinate data from visible surfaces in a scene. This data consists of image data points that explicitly represent scene surface geometry of each sampled point by utilizing range information for each point.
One approach to three-dimensional vision is the "time of light" approach which measures the range at given points by measuring the timing of the return of a pulse of light projected on a target. Unfortunately, the extreme electronic speed required for this approach severely limits the resolution of such systems. LIDAR systems avoid the problems of the time of light approach by modulating a light beam and comparing the modulation of the transmitted and returned signal to determine range. For instance, coherent frequency modulator (FM) LIDAR incorporates frequency modulation/mixing to determine range. However, FM LIDAR systems suffer from the limited frequency modulation capabilities of current laser diodes. Phase shift (AM) LIDAR is a preferred approach which correlates the phase shift between outgoing and incoming amplitude modulated light beams to determine range. As a result, each pixel in a collected image is an individual range measurement, and the resulting image is somewhat like a "terrain map" of the scanned area. When the range information is combined with the two-dimensional image, true three-dimensional vision is accomplished.
A number of difficulties are encountered in the opto-mechanical design of LIDAR systems. One is achieving an acceptable signal-to-noise ratio. For example, LIDAR scanning systems often utilize the same optical path for the transmitted and received light. However, small particles of dirt or scratches in the optical elements, or windows, can cause the transmitted light to be received directly back into the receive optics as noise or crosstalk.
Further, LIDAR systems frequently receive light from the entire scanned volume simultaneously. In these systems the field of view is the same as the scanned volume. This approach, however, yields a relatively small signal-to-noise ratio for a given output of light power. One alternative approach is to receive the reflected radiation via a scanning system so that the field of view of the receiving optical subsystem is very small. This increases the signal-to-noise ratio to permit better accuracy and to allow the use of a lower output light transmitter. Further, in such systems since the transmitted light beam is relatively narrow and the field of view of the receiver optics is narrow, it is easier to construct a system where the outgoing and incoming light beams are not coaxial and do not coincide. One approach, for example, uses dual rotating polygon mirrors, one for transmitting the scanning output beam and the other rotating polygon mirror being employed to receive the incoming reflected light. However, the dual rotating polygon approach presents alignment problems to ensure that the field of view is coincident with the transmitted light.
Thus, it is desirable to provide an improved three-dimensional imaging system which employs amplitude modulated LIDAR with an improved signal-to-noise ratio. Further, it is desirable to provide a LIDAR system in which the transmit and receive beams are not coaxial to reduce unwanted reflections and crosstalk. In addition, it is desirable to have a LIDAR system which employs a small field of view that is scanned along with the transmitted beam and which avoids the use of separate polygons that must be accurately aligned.
A LIDAR scanning system achieving the above-mentioned desirable features is provided according to this invention. The LIDAR scanning system includes a single rotating faceted polygon mirror for receiving and reflecting a transmitted light beam along a linear path as the polygon mirror rotates. Diffuse light reflected from the scanned surface is received by the rotating polygon mirror along an optical path at a small parallax angle relative to the transmitted optical path. The received light is reflected by the rotating polygon mirror and directed to a sensor. The transmitted and returned beams are reflected by adjacent facets of the polygon mirror. In this way, the field of view of the sensor is scanned and is coincident with the point on the object receiving the scanned light.
In accordance with another embodiment of the present invention, the scanning system also employs a tilting mirror which reflects both transmitted and received light for creating a linear scan and receive pattern. The tilting mirror creates scans along multiple positions of the Y-axis to generate a rectangular frame scan.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates, from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.